Azeotrope of 1,1,2-trichloro-1,2,2-trifluoroethane and nitromethane

ABSTRACT

AN AZEOTROPE OF ABOUT 97.1 WEIGHT PERCENT 1,1,2-TRICHLORO-1,2,2-TRIFLUOROETHANE AND ABOUT 2.9 WEIGHT PERCENT NITROMETHANE, USEFUL IN VAPOR DEGREASING AND CLEANING APPLICATIONS. THE PRESENCE OF NITROMETHANE INHIBITS REACTION OF THE 1,1,2-TRICHLORO-1,2,2-TRIFLUOROETHANE WITH ZINC TO PRODUCE UNSATURATED HALOCARBONS AND INHIBITS ALUMINUM CORROSION AS WELL AS STRESS-CORROSION CRACKING OF TITANIUM ALLOYS. MIXTURES WHEREIN THE NITROMETHANE IS PRESENT IN GREATER THAN AZEOTROPIC AMOUNTS ARE FLAMMABLE; WHILE MIXTURES WHEREIN THE NITROMETHANE IS MUCH LESS THAN AZEOTROPIC AMOUNTS ARE NOT AS EFFECTIVE WITH RESPECT TO MOST OF THE INHIBITING PROPERTIES OF THE AZEOTROPE.

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AZEOTROPE 0F l,l,Z-TRICHLORO-l,2,3-IRIFLUOROETHANE 'AND NITROMETHANEFiled Jan. 1.8, 1968 DISTILLATE g '5 z msmmz i 22.0 t v RESIIDUE 3RESIDUE .2. 3 0 I v L0 20 3.0

'1. NITROMETHANE CHARGED INVENTOR JAMES 6. sum

ATTORNEY United States Patent Oihce 3,573,213 AZEOTROPE F1,1,2-TRICHLORO-1,2,2-TRI- FLUOROETHANE AND NITROMETHANE James G. Burt,Oxford, Pa., assignor to E. I. du Pont de Nemours and Company,Wilmington, Del. Filed Jan. 18, 1968, Ser. No. 698,799 Int. Cl. C09119/00;C11d 7/50; C23g /02 US. Cl. 252-472 2 Claims ABSTRACT OF THEDISCLOSURE An azeotrope of about 97.1 weight percent1,1,2-trichloro-l,2,2-trifluoroethane and about 2.9 weight percentnitromethane, useful in vapor degreasing and cleaning applications. Thepresence of nitromethane inhibits reaction of the1,1,2-trichloro-1,2,2-trifluoroethane with zinc to produce unsaturatedhalocarbons and inhibits aluminum corrosion as well as stress-corrosioncracking of titanium alloys. Mixtures wherein the nitromethane ispresent in greater than azeotropic amounts are flammable; while mixtureswherein the nitromethane is much less than azeotropic amounts are not aseffective with respect to most of the inhibiting properties of theazeotrope.

BACKGROUND OF THE INVENTION (1) Field of the invention This inventionrelates to an azeotrope of 1,1,2-trichloro- 1,2,2-trifluoroethane andnitromethane, and to its use in vapor-phase degreasing or cleaning.

(2) Description of the prior art Vapor degreasing and cleaning has foundwidespread use in industry for degreasing and otherwise cleaningobjects, especially intricate parts.

In its simplest form, vapor degreasing or cleaning consists in loweringa room-temperature object to be cleaned into the vapors of a boilingsolvent. Vapors condensing on the part provide clean distilled solventto'wash away grease and other contamination. Final evaporation ofsolvent from the part leaves behind no residue as would be the caseWhere the part was simply washed in liquid solvent.

The solvent is contained in a bath called the boiling sump. Heat issupplied to the sump, usually electrically. A so-called vapor space,relatively free of drafts, in which parts are vapor-rinsed, is providedby extending upward the walls of the sump. The height at which vaporsare allowed to rise in the vapor space is controlled by the heat inputto the sump and by condensors, generally consisting of water-cooledtubes disposed around the inside periphery of the vapor space walls nearthe top. Frequently, the condensate from the condenser is collected andpassed through a separator to remove co-condensed water before thesolvent is returned to th sump.

In more complex variations of the above-described vapor degreasingoperation, a so-called clean sump, in addition to the boiling sump, isprovided. In this variation, which is more commonly used than is thefirst-described system, condensate from which co-condensed water hasbeen removed, is directed to a second sump upstream from the boilingsump. Overflow from the clean sump passes over a weir into the boilingsump. Some operators prefer to rinse parts first in the boiling sump or,more usually, in the clean sump before vapor rinsing. Obviously, shouldthe part become warmed to the temperature of the boiling solvent, vaporwill no longer condense on the part. In this event, the part is allowedto cool out side the hot vapor zone before vapor rinsing or a spray fromthe clean sump is provided by means of a pump.

Heat is ordinarily not provided to the clean sump so that rinsingtherein does not overly heat the part and final vapor rinsing ispossible without intermediate cooling. Occasionally, throughco-distillation of water with waterimmiscible solvents, vapor degreasersare also used to dry water-wet articles.

Such vapor degreasers and dryers are well known in the art. For example,Sherliker et al. in US. Pat. 3,085 918 disclose a vapor degreasercomprising a boiling sump, a clean sump, a water separator and otherancillary equipment. Sherliker discloses in US. Pat. 3,003,347 thedrying of water-wet articles in a vapor degreaser by codistillation ofwater with boiling chlorinated hydrocarbon solvents containing acationic surface-active agent.

A number of solvents have been used in vapor deg'reasers but none ofthem is ideal. For example, Stoddard solvent, a petroleum distillate, isusually considered unsatisfactory, not only because it is highlyflammable but also because, due to its relatively low molecular wegiht,its vapors tend to diffuse out of the degreaser.

Chlorinated hydrocarbons have also been employed. They are moresatisfactory with respect to the deficiencies of the Stoddard solvent;however, they are deficient in other respects. For example,trichloroethylene, 1,1,1-trichloroethane, and tetrachloroethylene areall moderately toxic in acute systemic toxic hazard by inhalation, asrated by Sax. Dangerous Properties of Industrial Materials, ReinholdPublishing Corporation, New York, N.Y., (1957). Carbon tetrachloride hasalso been considered for use but is severely toxic. Moreover, thechlorinated solvents tend in use to decompose by hydrolysis or oxidationin the presence of moisture, metals and oxygen to yield acidic materialsthus frequently corroding the vapor degreaser and metal parts beingcleaned. Despite the above-recited disadvantages, the chlorinatedhydrocarbons, especially trichloroand perchloroethylene, and1,1,1-trichloroethane have been widely used in degreasing applicationsby including with them a number of compounds and mixtures of compoundsto prevent decomposition of the chlorinated hydrocarbon in use. Forexample, Bachtel in US. Pat. 2,970,113 discloses the inhibition ofreaction between 1,1,1-trichloroethane and iron or aluminum, and withmineral oil, with mixtures comprising phosphines or alkyl phosphates.1,4-dioxane with and without nitromethane. Sims in US. Pat. 3,060,-discloses the stabilization of chlorinated hydrocarbon solvents such as1,1,1-trichloroethane with a mixture of a nitro aliphatic hydrocarbonsuch as nitromethane and an aliphatic carboxylic ester such as ethylacetate. In US. Pat. 3,113,155 Sims dicloses the mixture of US. Pat.3,060,125 with additionally 1,3-dioxolane and alkyl 1,3-dioxolanes. Simsdiscloses the use of the latter compositions in degreasing aluminum,copper, and iron in US. Pat. 3,113,156. However, in none of thesedegreasing applications has the chlorinated hydrocarbon been em.- ployedwith nitromethane alone.

It has been found that the halogenated hydrocarbon1,1,2-trichloro-1,2,2-trifiuoroethane is an effective and non-toxicsolvent useful in degreasing applications. However, it attacks reactivemetals such as zinc and aluminum and reduces the stress corrosion crackresistance of some titanium alloys. It is of economic consequence that1,1,2- trichloro-l,2,2-trifiuoroethane cannot be used in degreas ingapplications in which such metals and metal alloys are present. However,with the addition of nitromethane in an amount resulting in azeotropicmixture, it has been found that the reaction with such metals can beeffectively inhibited.

A mixture of trichlorotrifluoroethanes and 0.1% to 5% of amononitroalkane is known (see Kvalnes US. Pat. 3,085,116) for use ininhibiting the reaction of 1,1,2-trichloro-l,2,2-trifluoroethane withprimary or secondary alcohols; and Eiseman in US. Pat. 3,355,391discloses a degreasing mixture containing trichlorotrifiuoroethane whichis stabilized with respect to reaction with zinc metal by addingnitromethane and propargyl alcohol. However, both are silent withrespect to the recognition of the problem of inhibition of the reactionof the trichlorotrifiuoroethane with zinc to produce unsaturatedhalocarbons and to the problem of inhibiting aluminum corrosion andtitanium alloy stress-corrosion cracking. Moreover, neither recognizesthat azeotropic proportions of nitromethane with 1,1,2 trichloro 1,2,2trifiuoroethane are most effective in such inhibiting, and neitherrecognizes that mixtures of 1,1,2-trifiuoro-1,2,2-trichloroethane andamounts of nitromethane in excess of azeotropic amounts are flammable.Such flammability is not critical in the uses of the mixtures disclosedby Kvalnes but becomes of great importance in an open system, such as avapor degreasing or cleaning system.

SUMMARY OF THE INVENTION The composition of this invention comprises anazeotrope of 97.1 weight percent 1,1,2-trichloro-1,2,2-trifluoroethaneand 2.9 weight percent nitromethane.

The invention can also be described as an improved vapor degreasing orcleaning composition wherein the composition comprises a halogenatedhydrocarbon and an inhibitor in which the improvement comprises1,1,2-trichloro-1,2,2-trifiuoroethane as the halogenated hydrocarbon andnitromethane as the inhibitor in which the nitromethane is present in anamount of 2.9 weight percent based on the weight of1,1,2-trichloro-1,2,2-trifluoroethane.

One process aspect of this invention consists of a process forinhibiting the formation of unsaturated halocarbons caused by thereaction of 1,1,2-trichloro-1,2,2-trifluoroethane and zinc whichcomprises adding nitromethane to the1,1,2-trichloro-1,2,2-trifluoroethane until the mixture contains 2.9weight percent nitromethane and 97.1 weight percent1,1,2-trichloro-1,2,2-trifluoroethane, and contacting the mixture withzinc.

Another process aspect of this invention consists of a process forinhibiting aluminum corrosion or titanium stress-corrosion crackingwhich comprises adding nitromethane to1,1,2-trichloro-1,2,2-trifluoroethane until the mixture contains 2.9weight percent nitromethane and 97.1 weight percent1,1,2-trichloro-1,2,2-trifluoroethane, and contacting the mixture withaluminum or titanium.

DESCRIPTION OF THE DRAWING The graph of the drawing shows percentcomposition of the mixture of 1,1,2-trichloro-1,2,2-trifiuoroethane andnitromethane which constitutes the azeotropic proportions.

DESCRIPTION OF THE INVENTION As stated above, it has now been found thatthe reaction of 1,1,2-trichloro-1,2,2-trifluoroethane with zinc toproduce unsaturated halocarbons can be effectively inhibited both in theliquid and in the vapor phase by addition of small amounts ofnitromethane to the solvent, and that the corrosion of aluminum and thedecrease in stress-corrosion crack resistance of titanium alloys can .beeffectively inhibited by the same procedure.

It has further been found that nitromethane forms with1,1,2-trichloro-1,2,2-trifiuoroethane an azeotrope which contains enoughnitromethane (2.91 weight percent) to eifectively inhibit reaction withthe above-mentioned metals in both the vapor and liquid phase and whichis nonflammable under all use conditions.

With less than the azeotropic amount of nitromethane, the mixture is notas effective in preventing decrease in stress-corrosion crack resistancein titanium alloys and in amounts greater than the azeotropic amount,nitromethane confers flammability on the evaporating mixture.

The azeotropic mixture can be prepared by adding nitromethane to1,1,2-trichloro-1,2,2-trifluoroethane until the azeotropic proportionsare obtained or by distillation of mixtures of the two compounds havingpercent compositions outside the azeotropic range. The mixture can thenbe added directly to a vapor degreasing machine, as shown in some of thefollowing examples.

The examples which follow illustrate the invention in greater detail.

EXAMPLE 1 This example shows the minimum boiling point of the azeotropicmixture of the invention.

The temperature of the vapor over boiling mixtures of 1,1,2 -trichloro1,2,2 trifluoroethane was measured with correction to 760 mm. Hgessentially by the method described by W. Swietoslawski, p. 32,Azeotropy and Polyazeotropy, the Macmillan Company, New York, N.Y.,(1963), with the results as shown in the following Table I.

TABLE I Liquid mixture Parts by Wt., 1,1,2-trlchloro- Parts by wt.,Vapor temperature trifiuoroethane nitromethane (1 atm0sphere, C.)

The table shows that the minimum boiling point of the azeotrope is about46.77 C.

EXAMPLE 2 This example confirms the existence of the azeotrope of theinvention by measuring the percent composition of the distillate ofvarious mixtures of the azeotrope ingreclients.

The composition of the azeotrope was further confirmed by distillingvarious mixtures of nitromethane and 1,1,2-trichloro-l,2,2-trifluoroethane through a 3-foot glass helices-packedfractionating column with a known efficiency of 30 plates. Atequilibrium small samples of distillate and residue were analyzed in acalibrated vaporphase chromatograph after /3 and after /3 of the samplehad been distilled. The results of analyses were plotted in the graphshown in the accompanying drawing which shows the crossover pointcorresponding to the composition, cited in Example 1, of the azeotrope.In the graph, the percent of nitromethane found in the distillate (e.g.,distillate /3) and the amount of nitromethane found in the residue(e.g., residue) /3 after the distillation are plotted against thepercent of nitromethane charged to the mixture.

EXAMPLE 3 This example confirms that the azeotrope of this inventionexists in its azeotropic proportions during use in a degreasingoperation.

In order to approximate the conditions of recovery of used solvent froma vapor degreaser, a mixture of 7.5 grams of diisoctyladipate, 17.5grams of colorless mineral oil, and 225 grams of the azeotropic mixturewas distilled in an essentially l-plate still. The solvent was recoveredin 91 Wt. percent (205 grams) yield. The recovered solvent was analyzedby gas chromatography yielding the values 97.03 wt. percent,1,1,2-trichloro-1,2,2-trifluoroethane and 2.97 wt. percent nitromethane.

EXAMPLE 4 This example describes the critical solution temperature ofthe azeotrope of this invention.

The critical solution temperature, i.e. that temperature below whichseparation of phases occurs, was determined by cooling mixtures ofnitromethane and 1,1,2-trichloro- 1,2,2-trifluoroethane until cloudinesswas observed. The results are shown in the following Table II.

EXAMPLE This example shows the effectiveness of the azeotrope of thisinvention in inhibiting metal corrosion in the liquid phase, the vaporphase and at the interface.

Pure 1,1,Z-trichloro-l,2,2-trifluoroethane, the azeotrope, and acommercial stabilized trichloroethylene sold under the trade nameTriclene were refluxed separately and continuously in a simplegalvanized vapor degreaser consisting of a cylindrical vessel of about20 inches diameter and 3 feet depth with a water cooled coil installedabout inches about the surface of the boiling solvent. In all cases asmall amount of water was floated on top of the solvent to approximateco-condensed water in use. Fresh solvent and water were added as neededto replace evaporation losses. The following Table III describes theeffect of each solvent upon the galvanized container.

TABLE III.CORROSION OF A GALVANIZED DEGREASER Days of operation withoutcorrosion Remarks Solvent 1,1,2-trlchloro-1,2,2-trifluoroet ane.Stabilized trichloroethylene.

Azeotrope 1 General attack on zinc in the vapor phase. 37 Corrosion atliquid surface and just above.

100 Terminated without serious corrosion in the liquid and the vaporphase. Some dark spots at liquid surface level.

EXAMPLE 6 This example shows the effectiveness of the azeotropiccomposition of the invention compared with other proportions of theingredients and with other inhibitors.

The inhibition of corrosion of ,zinc was further tested by placing agalvanized steel strip in screw capped bottles containing two drops ofwater, 100 grams of 1,1,2-trichloro-1,2,2-trifluoroethane andnitromethane in the below-indicated concentration-s. The closed bottleswere stored at 38 C. until corrosion was evident, generally, at theinterface and in the vapor phase. Other inhibitors useful in someapplications were also tried without evident success. The results of thetests shown in the following Table IV indicate that enduring inhibitionis achieved with nitromethane only in concentrations of 2-3 wt. percent.

TABLE IV.-INHIBITION OF CORROSION ON ZINC Days to fit InhibitorConcentration 1 corrosion Dioxane Welght percent based on 1, 1,2-trlchlor0-1, 2, Z-trifiuoroethane.

6 EXAMPLE 7 The following experiment demonstrates that nitromethane isnot depleted by reaction with zinc surfaces in the liquid phase underthe conditions of a vapor degreaser. A mixture of1,1,2-trichl0ro-1,2,2-trifluoroethane and 0.5 wt. percent watercontaining a level (1.0 wt. percent) of nitromethane deliberately lowerthan that of the azeotrope was refluxed for 21 days in the presence of asample of submerged galvanized window screen corresponding to 10,000square inches of zinc surface. At the end of this period no visiblecorrosion had occurred and by gas chromatographic analysis thenitromethane concentration was 1.1 wt. percent.

EXAMPLE 8 Using the method of B. F. Brown, Materials Research andStandards 6, No. 3, 129-133 (March 1966), modified slightly in that thesolvent also applied to the specimen at the boiling point, this exampleshows that 1,1,2- trichloro-1,2,2-trifluoroethane,1,1,1-trichloroethane, and trichloroethylene promote the stresscorrosion cracking of the titanium alloy, Ti-5 Al-2.5 Sn. Table Vfollowing shows that 0.75 wt. percent nitromethane is insufi'icient toprevent notable reduction in fracture toughness, whereas the azeotropicamount of nitromethane is suflicient entirely to prevent such reduction.

TABLE V Loss of toughness in Ti-S Al-2.5 Sn alloy Nitromethane cone. inFracture toughness 1,l,2-trichlorotrifluoroethane, reduction in (wt.percent) percent of original 0 57 0.75 50 2.92 0

EXAMPLE 9 In a manner analogous to Example 6, aluminum alloy No. 6061,Al-1.0 Mg-0.6 Si-0.25 Cu-0.25 Cr (Metals Handbook, vol. 1, Amer. Soc.for Metals, 8th edition (1961)), in small samples was exposed to varioussolvents at 165 F. for 28 days in sealed glass tubes under autogenouspressure in the presence of about 3 wt. percent water based on thesolvent weight. The aluminum corroded substantially in all cases exceptwhere the solvent was the azeotrope. The percent decomposition of thesolvent was estimated by determination of generated chloride ion. Thelower limit of chloride ion detection corresponded to one one-thousandthpercent solvent decomposition. Table VI describes the results.

TABLE VI Corrosion of aluminum alloy (165 F./28 days) Solvent: Percentsolvent decomposition 1,1,2-trichlorotrifiuoroethane 0.56. Azeotrope Notdetectable. Commercial inhibited trichloroethylene 0.05. Commercialinhibited methyl chloroform 0.81.

EXAMPLE 10 This example shows the separability and flammability ofnon-azeotropic proportions of the ingredients.

A mixture of 5 wt. percent and sufiicient 1,1,2-trichloro-1,2,2-trifluoroethane to make wt. percent mixture 'was cooled to 40 F.(4.4 C.), a temperature frequently encountered in a degreaser inpractice. A nitromethanerich layer separated and floated on the top inthe water separator. The layer was flammable.

Tt follows from the data of Example 4 that separation of anitromethane-rich layer will occur under a variety of conditions above 0C.

7 EXAMPLE 11 This example shows the flammability of non-azeotropicproportions of the ingredients.

A mixture of 4.3 wt. percent of nitromethane and sufficient1,1,2-trichloro-1,2,2-trifiuoroethane to result in 100 wt. percentmixture was spilled on a concrete floor at room temperature in thepresence of a flame. When the mixture was nearly evaporated, the vaporsignited and burned briskly in a blue flame.

The azeotropic mixture is nonfiamma-ble. Since it evaporates unchangedin composition, no amount of evaporation can produce flammable vapors.On the other hand, any excess of nitromethane above the azeotropicconcentration, no matter how slight, will produce a flammablecomposition on evaporation. The hazard is especially severe in the caseof a slow dripping leak whereby on evaporation of the solvent, residualnitromethane would be concentrated in a small area. Likewise, partsdrying in air would produce flammable conditions.

EXAMPLE 12 This example demonstrates that attack on galvanized iron(zinc) by 1,1,2-trichloro-1,2,2-trifluoroethane, under conditionsanalogous to those of a vapor degreaser (reflux temperature, presence oftraces of water and a zinc salt (ZnCl occurs as follows:

and that nitromethane in azeotropic amount inhibits the reactioncompletely within the limits of detection of CTFE.

To each of two 500 cc. round-bottom flasks fitted with C. tap watercooled reflux condensers and solid carbon dioxide/acetone-cooled trapsattached by means of hoses to the top of the refiux condensers, wascharged:

To flask No. 1 was also added:

Nitromethane (11.4 grams, 2.9 wt. percent).

Both flasks were refluxed for 48 hours after which the metal specimenswere removed and the contents of the traps were transferred for analysisto evacuated gas sample bulbs.

The totally immersed metal specimens were unchanged in both flasks. Thelonger strip in flask 1 was slightly discolored at the liquid-vaporinterface. In flask No. 2, that part of the longer strip in the vaporphase was heavily corroded and was covered with a black granular solid.

The contents of the sample bulbs were analyzed in a model 720 F and MScientific Corporation gas chromatograph using a S-meter column packedwith 20 wt. percent Dow-Corning DC-200 silicone oil on Chromosorb W(Johns-Manville) support. The helium sweep rate was cc. min. Thetemperature of the column was programmed; 200 sec. at room temperature,then 25 C. min. to 230 C.

1,1,2-trichloro-1,2,2-trifluoroethane was identified by retention time(574 to 596 seconds) in both samples.

Chlorotrifluoroethylene, identified by 214 second retention time, wasfound in bulb No. 2 in an amount corresponding to 5.53 area percent[area under CTFE peak+sum of the areas under all peaks in a mixturecontaining largely air and 1,1,2-trichloro-1,2,2-trifluoroethane.Analysis of the contents of bulb No. 1 showed no compound of retentiontime about 214 seconds. The estimated sensitivity of the method was suchthat at least as little as 0.001% chlorotrifluoroethylene would havebeen detected.

The foregoing detailed description has been given for clearness ofunderstanding only and no unnecessary limitations are to be understoodtherefrom. The invention is not limited to the exact details show anddescribed, for obvious modifications will occur to those skilled in theart.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. An azeotrope consisting essentially of 97.1 weight percent1,1,2-trichloro-1,2,2-trifluoroethane and 2.9 weight percentnitromethane.

2. In the process for cleaning zinc, aluminum or titanium metal articleswith 1,1,2-trichloro-1,2,2-trifluoroethane as the solvent, theimprovement which consists of adding 2.9 weight percent of nitromethaneto the solvent to form an azeotrope mixture therewith, and applying saidmixture to the metal whereby corrosion thereof is prevented.

References Cited UNITED STATES PATENTS 3,355,391 11/1967 Eiseman, Jr.

FOREIGN PATENTS 627,411 9/1961 Canada 252-171 LEON D. ROSDOL, PrimaryExaminer W. E. SCHULZ, Assistant Examiner US. Cl. X.R.

